FluidPhase~qquilibria,40 (1988) 113-125 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands 113 VAPOR PRESSURES OF N-BUTANE, DIMETHYLETHER,METHYL CHLORIDE,METHANOLAND THE VAPOR-LIPUID EQUILIBRIUM OF DIMETHYLETHER - METHANOL: EXPERIMENTAL APPARATUS, RESULTSAND DATA REDUCTIDN H. HOLLOORFF and H. KNAPP Institute of Thermodynamics and Plant Design, Technical University Berlin (F.R.G.1 (Received July 27, 198’7; accepted in final form November 30, 1987) ABSTRACT Holldorff, H.andKnapp, H., 1988. Vapor pressures of n-butane, dimethyl ether, methyl chloride, meth- anol and the vapor-liquid equilibrium of dimethyl ether-methanol; experimental apparatus, results and data reduction. Fluid Phase Equilibria, 40: 113-125. A static equilibrium apparatus for experimental investigations of vapor- liquidand vapor-liquid-liquid phase equilibria at pressures10 kPa < p < 1 MPa and temperatures 250 K < T < 350 K was designedand built. The vapor pressureof n-butane was measured at 258 K < T < 357 K in order to test the the operability and accuracyof the entire system. Vapor pressures of dimethyl ether, methyl chloride and methanol were measured;Vapor-liquid equilibria in the binary mixture of dimethylether and methanolwere investigated. The experimental resultswere reduced. INTRODUCTION The design, selection, or optimization of separation processesin chemical production plants requires information on phase equilibria. In many cases It 1s not possible to predict the conditions in multicomponent and multiphase systems with sufficient accuracyand certainty. Our knowledge in molecular theory and our models of liquid solutions are still imperfect particularly for strongly interacting, highly polar or assoclatlng molecules. The required informatlon must be obtained in the experiment. Experimental data are costly and only sporadic but are essential for responsible process design and for further improvement of theory. 037%3812/88/$03.50 0 1988 Elsevier Science Publishers B.W.
Transcript
FluidPhase~qquilibria,40 (1988) 113-125 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
113
VAPOR PRESSURES OF N-BUTANE, DIMETHYL ETHER, METHYL CHLORIDE, METHANOL AND THE
VAPOR-LIPUID EQUILIBRIUM OF DIMETHYL ETHER - METHANOL:
EXPERIMENTAL APPARATUS, RESULTS AND DATA REDUCTIDN
H. HOLLOORFF and H. KNAPP
Institute of Thermodynamics and Plant Design,
Technical University Berlin (F.R.G.1
(Received July 27, 198’7; accepted in final form November 30, 1987)
ABSTRACT
Holldorff, H. and Knapp, H., 1988. Vapor pressures of n-butane, dimethyl ether, methyl chloride, meth- anol and the vapor-liquid equilibrium of dimethyl ether-methanol; experimental apparatus, results and data reduction. Fluid Phase Equilibria, 40: 113-125.
A static equilibrium apparatus for experimental investigations of vapor-
liquid and vapor-liquid-liquid phase equilibria at pressures 10 kPa < p < 1 MPa
and temperatures 250 K < T < 350 K was designed and built.
The vapor pressure of n-butane was measured at 258 K < T < 357 K in order to
test the the operability and accuracy of the entire system.
Vapor pressures of dimethyl ether, methyl chloride and methanol were measured; Vapor-liquid equilibria in the binary mixture of dimethyl ether and methanol were investigated. The experimental results were reduced.
INTRODUCTION
The design, selection, or optimization of separation processes in chemical production plants requires information on phase equilibria. In many cases It 1s not possible to predict the conditions in multicomponent and multiphase systems with sufficient accuracy and certainty. Our knowledge in molecular theory and
our models of liquid solutions are still imperfect particularly for strongly interacting, highly polar or assoclatlng molecules.
The required informatlon must be obtained in the experiment. Experimental
data are costly and only sporadic but are essential for responsible process
This report is the first of a series of reports on results of an experimental
study of phase equilibria of mixtures containing CHJOCH3, CH,Cl, CH>OH and HZ0
(see Table 1). These components can be found for example, in plants producing methyl ceilulose and must be separated for varies reasons, such as recovery and waste water treatment.
TABLE 1
Experimental program
Substance pLv/kPa 1 2 min max
%"bO 128 1073
CH,Cl 124 1098
CH,OH 13 243
%"6O CH,OH 35 1078
TLV/K Pts min max
254 321 19
25& 322 29
293 362 15
2% 353 72
EXPERIMENTAL METHOD
At high vapor pressures, with components of very different volatilities, in
the presence of two liquid phases it is difficult to take representative liquid
samples for chemical analysis. It is advantaqous to use a static cell and
prepare the mixture gravimetrically or volumetrically. The static method is one
of the oldest methods used to study phase equilibria (Magnus 1836). Recent years
have shown an increasinq interest in this method, and a variety of apparatuses
have been described in literature (e.g. Van Ness and Gibbs 1972, Pemberton and Mash 1978, Aim 1978, Kolbe and Gmehling 1985).
EXPERIMENTAL EQUIPMENT
The static equilibrium cell is shown in figure 1. It is designed for maximum
pressures of 1.5 MPa. The cell is made of a thick-walled (9 nnn) Doran glass
cylinder (H = 100 mm, Di = 81 mn. V = 512 cm') (1) closed on both sides by
stainless steel (W 1.4571) cover-plates (2, 3) with Teflon as gasket material
(4). The cell is filled 9/10 with liquid. Corrections of the liquid composition due to partial vaporization are therefore minimal. However there is still a
vapor inventory large enough to allow withdrawal of samples without affecting
pressure and composition.
An excentric hollow-shaft stirrer (5) sucks vapor and liquid from the surface
and distributes them near the bottom. The stirrer shaft is sealed by a teflon -1 packing (6) and mercury (7) and is driven by an electric motor at 200 min .
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Fig. 1. Equilibrium cell
The Liquid components are injected into the cell through
are inserted through teflon-coated septa (8). The temperature
in a therm0 well (9).
The complete system is shown schematrcally in Figure 2.
capillaries, which
sensor 1s inserted
The glass equilibrium cell (1) is immersed in a double wall glass vessel (2)
containrng 40 1 of a water-glycol mixture, temperature controlled within ~0.02 K
by a circulation thermostat (3) (Haake F3-k). The thermostat has an operating
range of 250 K < T < 360 K. The cell can be viewed through a window (4). The
interspace 1s purged with dry nitrogen to prevent fogging at low operating
temperatures. Different methods are used to fill substances into the cell: low
volatile liquids (HzO, CH,OH) are carefully degassed and then transferred from
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Fig. 2. Schematic diagram of the static phase equilrbrium apparatus.
storage flasks (6) by diaphragm type dosrmetric pumps (5). High volatile
substances (CZH6U, CH,Cl) are distilled From the high pressure flasks (7) into
the refrrgerated cell. Liquid samples can be withdrawn through caprllaries (81,
evaporated in needle valves (VI and collected as vapor in sample flasks. The
entire system can be evacuated by a vacuum pump (15).
Instrumentation and control
The temperatures in the bath and equilibrium cell are measured by PtlOO
temperature sensors (101 in combination with a digrtal indicator (Systemteknik
51223). The system was calrbrated against a standard resistance thermometer
(Rosemount 162 CE).
The cell is connected to one side of a differential pressure gauge (Hdttxrger
Baldwin PDl/O.l) (11) whrch is located directly above the cell and heated to
9B°C to prevent condensation and to thermostat the inductive PD-transmitter. The
differential pressure indicator-controller c&p 210 kPa) is used to adjust the
reference pressure by adding or venting nitrogen (121. The reference pressure is
read on an aneroid precision pressure indicator (13) (Wallace & Tiernan 61-1500)
or set by a pressure balance (14) (Oesgranqes 6 Huot 3030-Zl-PR).
The mass of each component filled into the cell is determined gravrmetrically
on a precls~on scale (m 21 mg) by weighing the storage flasks (6 and 7) before
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and after filling. The composrtion of the liquid phase can be calculated after
correcting for the portlon contained in the vapor fraction.
The vapor composition can be determined by a gaschromatograph (Hewlett
Packard 5830 A) with a thermal conductivity detector calibrated with reference
mixtures.
Table 2 lists the sensitivity of the instruments or systems and their
estimated inaccuracy, resulting from calibration, fluctuation, handling, etc..
TABLE 2
Sensitivity and inaccuracy of the instrumentation
Variable Instrument or Method Sensitivity Inaccuracy
T (K) resistance thermometer 0.01 0.04
Ap (kPa1 differential pressure gauge 0.01 0.05
p (kPal aneroid manometer 0.02 0.06
p (kPa) pressure balance 0.01: 0.1%
x (mol/molj gravimetrical 0.00001 0.0001
y (mol/mol) gaschromatograph 0.00001 0.003~0.01’
* dependent on component and concentration range
SUBSTANCES
The water used is deionised and double distilled. Methanol was supplied by
Merck with a purity of min. 99.5% and a maximum water content less than 0.01%.
The inert gases were removed from both liqurds by partial evaporatian under
vacuum for 15-30 minutes.
Dimethyl ether and methyl chloride were provided by Henkel KGaA. Methyl
chloride showed no measurable impurities in gaschromatographic analysis on
Porapak Q and Carbowax 1540 columns except N2. It was degassed by repeated
freezing, evacuating the gas phase, and thawing.
Several impurities. mainly COz, were detected by gaschromatographic analysis
of the dimethyl ether. They were removed by distlllatlon. The degasslng
procedure was the same as used for methyl chloride. The final purity was
estimated to be 99.8%.
N-butane was chosen as a test substance. According to the suppliers
specifications, the purity was better than 99.95%
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EXPERIMENTAL RESULTS
Test measurement
The vapor pressure curve of n-butane was measured, to check the functioning
of the complete system and the accuracy of the instrumentation. The results are
shown in Table 3. The Antoine and Frost-Kalkwarf parameters are presented in
Table 5. The scattering of the experimental points can be seen in Figure 3, as
well as the deviations to vapor pressure equations recommended by other authors.
s 0.60
255 270 280 290 300 320 340 360
temperature T . K
fig. 3. Relative pressure deviation of measured data l and
different authors (OHaynes and Goodwin, 1982;v Kratzke et
al., 1977; 0 Boublik et al., 1973) compared to the vapor
The derivatives at constant temperature were calculated by using the Van Laar
model for the activity coefficients and the virial equation foe the fugacity
coefficients. The parameters in the Van Laar model were fitted with y taken I from the consistency test.
For inter- and extrapolation, the isosteric vapor pressure data were
correlated by equation (2). The corresponding vapor compositions were correlated
as a function of temperature by
In y1 = A + El / (T/K) + C ln(T/K) (6)
Fig. 4 shows the isosteric vapor pressure curves for the system CZH60-CHJOH.
In Fig. 5 examples of p-x,y diagrams are presented for T = 253, 293 and 323 K
(with data points of Chang et al. 11982) at 293 K). In Fig. 6 examples of T-x,y
diagrams for p = 1, 2 and 5 bars are shown.
/ .
*2l 1 I. I . , , , . 250 260 270 290 310 no 350 370
tomprature T . K
Fig. 4. Vapor pressure of dimethyl ether - methanol mixtures (0 x1=0.000, v x1=o.l227, 0 x1=o.2a54, 0 x1=o.4a39, rxl=0.752a, n x1=0.8718, 0 ~~=~.oooI
1000
; aw .
5. 600
z i !j 400 4.
200
0 0.0 a.2 0.4 0.6 0.n 1.0
380
360 - -. Y - 340 -
c
- 320 - ,; : : 300 - :
280 -
260 -
240 - 0.0
ccocentr.tion x, l d y, . wl/mol ccmcmtration xl a-d 7, . molfmol cngn m c&u (11 MQi121 c&Y I11
Fig. 5. p-x,y diagram For the system Fig. 6. T-x,y phase diagram the system CH,OCHJ-CH30H at l 253.15 K, r293.15 K, CH,OCH,-CH,OH at l 100 kPa, T 200 kPa, n 323.15 K, v data at 293.15 K measured m 500 kPa, the lines are the results by Chang et al.(lY&?),the lines are the OF Legendre polynomial Fits. results of Legendre polynomial fits
DATA REDUCTION
Thermodynamic consistency test
Whenever T,p,x and y were measured a consistency .test can be and was
performed as recommended by Fredenslund et al. (1977). The excess G'ibbs energy
was represented with a third degree Legendre polynomial. The required Fugacity
coefficients of the vapor phase
P Vi = exp ( ( 2 E Yj Bij - r r Yi Yj Bij 1 - 1
j ij RT (71
were calculated with second virial coefficients 8 estimated by the method of 1J
Hayden and O’Connell (1975).
Table 7 lists the average absolute deviations between experimental and calculated values in vapor composition AADy and in vapor pressure AADp. The
deviations are practically within the experimental inaccuracies.
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Models for the excess Gibbs energy
Binary parameters of several popular qE-models were fitted to isothermal
p,x,y-data sets, applying a maximum likelihood method (Prausnitz et al. 19ElO).
The virlal coefficients were estimated as mentioned above. The parameters of the
IJNIQUAC model were fitted for the original model and also for a modified UNIQUAC
model, as suggested by Prausnitz (1980) for alcohols and water.
The resulting parameters and the deviations irl total pressure are listed in
Table 8. The Wilson model and the modified UNIUUAC model can best represent the
Haynes, W.M., Goodwrn, R.D., 1982. Thermophysical Properties of Normal Butane from 135 to 700 K at Pressures to 70 MPa. National 8ureau of Standards Monography 169, Washrngton.
